During Phase II metabolism, an endogenous or exogenous substrate is rendered more hydrophilic through the covalent attachment of an endogenous molecule. Phase II is also referred to as conjugative metabolism, since conjugating moieties such as sulfonates, glucuronates, glutathiones, glycinates, acetates, and methylates, can be formed. Usually, Phase II metabolism is used by the body to increase the hydrophilicity of the substrate molecule, which facilitates transport and elimination of the conjugate product.
Phase II glucuronidation reactions are catalyzed by the UDP-glucuronosyltransferase (UGT) family of enzymes. The glucuronidation reaction consists of the transfer of the glucuronosyl group from uridine 5′-diphospho-glucuronic acid (UDP-GA) to substrate molecules that contain oxygen, nitrogen, sulfur or carboxyl functional groups. UGT enzymes represent a highly responsive defense system against the mutagenicity of carcinogens and the toxicity of both xenobiotics and endogenous metabolic intermediates. Also, in metabolically active tissues, certain transcription factors, such as the peroxisome proliferator-activated receptors (PPARs) play an active role in the regulation of UGT gene expression and activity. See, Runge-Morris, et al., PPAR Res. (2009), Article ID 728941, 14 pages, “Regulation of Sulfotransferase and UDP-Glucuronosyltransferase Gene Expression by the PPARs” (Hindawi Publ. Co., New York).
Glucuronidation plays a physiological role in the modulation of biologically active endogenous hormones and metabolic intermediates. One important endogenous UGT substrate is bilirubin, the metabolism of which is tightly controlled by UGT1A1, thus forming the elimination product bilirubin diglucuronide. Another isoform, UGT2B4, is known to be the major bile acid conjugating UGT enzyme in human liver, catalyzing glucuronidation of hyodeoxycholic acid. Further, it has been observed that various isoforms of UGTs are inducible, and the regulation of their activity may be an important determinant of drug detoxification and elimination. For example, PPARs serve an important role in the regulation of UGTs (Barbier, et al., J. Biol. Chem. (2003) 278:32852-32860).
The PPAR nuclear receptor network represents a central determinant of cellular energy balance. In heterodimeric partnership with the retinoid X receptor (RXR), PPAR forms a ligand-activated nuclear receptor transcription factor that is capable of integrating the expression of a wide spectrum of target genes (including UGTs) involved in cellular lipid metabolism, energy homeostasis, and inflammation. PPAR-alpha isoform expression is most prominent in the liver, kidney, and heart where it is engaged in the regulation of fatty acid oxidation. PPAR-alpha can also mediate induction of biotransformation enzymes. Using the key-lock analogy for enzyme receptor activation binding, fatty acids represent a major source of cellular energy and are important physiological activators of PPAR-alpha. The aforementioned PPAR-RXR heterodimer, in association with other co-activators, binds to DNA sequences called peroxisome proliferator response elements (PPREs) in the regulatory region of a target gene, initiating transcription and translation of enzymes, such as UGTs, and the like.
Therefore, in order to regulate the expression and activation of UGTs, one must provide an appropriate and selective enzyme inducer. Certain UGT inducers in humans are known, such as clofibrate, which is a PPAR-alpha agonist (Barbier, loc. cit.). In rats, UGT1A1 is a PPAR-alpha target gene, and exposure to certain inducers such as PPAR-alpha agonists can increase mRNA expression of this UGT in the liver (Shelby, et al., Drug Metab. and Disposition (2006) 34:1772-1778).
Furthermore, the UGTs have been shown to glucuronidate and inactivate 12-hydroxyeicosatetraenoic acid, or 12(S)-HETE (“12-HETE”). See, Turgeon, et al. J. Lipid Res. (2003) 44:1182-1191. As a response to certain inflammatory processes, and also UV-induced damage and/or skin carcinogenesis, arachidonic acid (“AA”), a fatty acid naturally present in membrane phospholipids, is metabolized by lipoxygenases (LOX) to a number of active eicosanoids. LOX metabolism of AA leads to the production of leukotrienes and hydroxyeicosatetraenoic acids (HETE). 5-LOX is responsible for the production of leukotrienes and 5-HETE, while 12-LOX yields 12-HETE. Specifically, lipoxygenase converts AA to the unstable hydroperoxy-eicosatetraenoic acid (HPETE), which is then hydrolyzed by peroxidase into HETE. Mounting evidence has shown that both 5-LOX and 12-LOX metabolites promote carcinogenesis through resistance to apoptosis as well as increased proliferation, angiogenesis and cell migration. Both 5-LOX and 12-LOX, which are largely absent from normal epithelia, are often constitutively expressed in various epithelial cancers.
Elevated levels of 12-LOX mRNA has also been linked to late stage cancer and poor prognosis. Additionally, 12-LOX has been demonstrated to play a direct role in skin carcinogenesis in mouse models, and 12-HETE has been detected at elevated levels in skin tumors compared to normal skin in mice. Specifically, levels of 12-HETE were 50-fold higher in papillomas and squamous cell carcinomas than in normal skin from the same mouse (Virmani, J., et al., Cancer Lett. (2001) 162(2): 161-165; Krieg, P., et al., Mol. Carcinog. (1995) 14(2): 118-129). Another mouse model demonstrated that use of a specific 12-LOX inhibitor (Baicalin) was protective against UVB-induced DNA damage in the skin (Bing-Rong, Z., et al., Photodermatol. Photoimmunol. Photomed. (2008) 24(4): 175-182). This result further underscores the importance of 12-HETE in skin cancer progression. Thus, without intending to be bound by theory, it is hypothesized that human skin cancer may be treated or prevented through inhibition of 12-LOX directly, or by inhibition of 12-LOX signaling, namely, by reduction in the 12-HETE levels. As discussed above, UGTs can glucuronidate, and thus inactivate 12-HETE, leading to its elimination by the body. However, UGTs expressed in the skin can be down-regulated by UV radiation, which is a prime suspect causative agent in skin cancers and other proliferative disorders.
In view of the above, it would be desirable to provide an enzyme inducer or agonist that can activate or induce expression of UGTs by administration of said inducer to an individual, animal or human. Further, it is expected that an enzyme inducer or agonist that can activate or induce expression of UGTs would also reduce levels of 12-HETE, which would serve as a useful contribution to the art.
In addition, use of an enzyme inducer or agonist for treatment of an individual for a cellular proliferative disorder, including skin cancer, comprising administering to the individual in need of such treatment a therapeutically effective amount of the compound pterostilbene wherein UDP-glucuronosyltransferase (UGT) activity is increased, would represent a useful contribution to the art.